1. Field of Invention
The present invention relates to the field of fuel cells, and more particularly in the field of polymer membrane fuel stacks for use in a proton exchange membrane fuel cell.
2. Description of Related Arts
A fuel cell is an energy source that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is most commonly used as the fuel for these types of cells. One of the most representative embodiments of this fuel cell technology is the proton exchange membrane (PEM) fuel cell. This kind of fuel cell is comprised of a membrane electrode assembly (MEA) which is further comprised of a polymer electrolyte membrane which is sandwiched between two catalyst coated papers, which correspond to an anode and cathode. The membrane electrode assembly (MEA) is then sandwiched between a pair of flow field plates which direct the fuel and oxidant respectively. The fuel cell operates according to the following steps: hydrogen fuel is channeled through a field flow plate to the anode on one side of the fuel cell, while oxidant is channeled through a flow field plate to the cathode on the other side of the cell; a platinum catalyst is located on the anode side which causes the hydrogen to split into positive hydrogen ions and electrons; the polymer electrolyte membrane allows only positively charged ions to pass through into the cathode, while the negatively charged electros must travel along an external circuit to the cathode, wherein a electrical current is created; at the cathode, the electrons and positively charged hydrogen ions combine with oxygen to form water as the only product which is outputted from the cell. Additionally, as the oxygen is blown through the cathode flow field plate channels it also cools down the fuel cell. The cathode flow field plate furthermore may be exposed to the atmosphere in which case it is considered an “open cathode structure.”
Conventional cathode flow field plate design is embodied as a saw or square-wave shaped structure in which air can be blown through by a blower or a fan. In comparison to water-cooled stacks, air-cooled stacks have a simpler balance of plan and an easier control strategy, and can be started up instantly.
One of the main flaws in an air-cooled proton exchange membrane fuel cell with polymer electrolyte membrane is thermal and water management. The polymer electrolyte membrane needs to be well hydrated in order to keep the internal electrical resistance of the membrane low. When air blows through the flow field plate channels, it cools the stack down, but also accelerates water evaporation leading to reduced water content in the membrane. Therefore, the fan speed needs to be carefully controlled (control strategy) according to current, ambient temperate, and relative humidity so that a balance can be reached. An inappropriate fan speed would reduce output power of the stack.
Another limitation of the air-cooled proton exchange membrane with polymer electrolyte membrane is hydrogen leaking. In the conventional design, the saw-side of the cathode flow field plate faces the membrane electrode assembly, which is comprised of the polymer electrolyte membrane sandwiched between the catalyst layers. Therefore, only the saw teeth are pressed onto the gasket, the rest of the areas are potential weak points for hydrogen leakage. The design usually limits the hydrogen working pressure to less than 0.5 bar·g. However, higher hydrogen pressure can help improve the kinetics, cell uniformity, response to load change and reduce the probability of hydrogen starvation which is extremely detrimental to fuel cell durability, but pressure above this level may cause a leak or gasket to burst.
FIG. 1 is perspective view of the prior art of a flow field plate assembly of a proton exchange membrane fuel cell. The cathode flow field plate A10 has a saw side A11 forming a plurality of channels A12 thereat and contacting with the MEA. As the cathode flow field plate A10 is used for the MEA, these channels A12 allow for air to be channel to help dissipate heat generated by the reaction process. To cope with greater heat strain the amount of air channeled through these channels A12 must be increase which lends to evaporation of the water, thereby increasing the electrical resistance in the MEA.
To create a seal for the channels A12, a gasket A20 is layered on the side of the saw side A11 of the cathode flow field plate A10 that a tip of the channel wall are extended to couple with the gasket A20, wherein the gasket A20 is sandwiched between the saw side A11 of the cathode flow filed plate A10 and the MEA.
FIG. 2 is a close-up perspective view of the cathode flow field plate assembly of a prior art of a proton exchange membrane fuel cell. This figure illustrates the weak points associated with the prior art of a proton exchange membrane fuel cell. The seal between the gasket A20 and the cathode flow field plate A10 is dependent upon the compression between the gasket A20 and the cathode flow field plate A10. Areas between the gasket A20 and the cathode flow field plate A10 where the tips of the channel walls come into contact with the gasket A20 are areas with a strong seal and when compression is increased the seal strength is increase allowing for a higher working pressure of the proton exchange membrane fuel cell. Areas with flow channel A12 are weak point WP areas due to the fact that there is no contact between the cathode flow field plate A10 and gasket A20, and thus there is no way for these areas to have their seal strength increased. When this seal strength between the tip of the channel wall of the cathode flow field plate A10 and the gasket A20 is enhanced leakage between the flow field plate A10 and the gasket A20 is prevented allowing for a higher working pressure and therefore improved fuel cell performance.